An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes in some cases. Generally, for example, at least one of these electrodes is transparent. OLEDs (based on polymers and/or evaporable small molecules) sometimes are used in television screens; computer monitors; small or portable system screens such as those found on mobile phones and PDAs; and/or the like. OLEDs may also sometimes be used in light sources for space illumination and in large-area light-emitting elements. OLED devices are described, for example, U.S. Pat. Nos. 7,663,311; 7,663,312; 7,662,663; 7,659,661; 7,629,741; and 7,601,436, the entire contents of each of which are hereby incorporated herein by reference.
A typical OLED comprises two organic layers—namely, electron and hole transport layers—that are embedded between two electrodes. The top electrode typically is a metallic mirror with high reflectivity. The anode is typically a transparent conductive layer supported by a glass substrate. The top electrode generally is the cathode, and the bottom electrode generally is the anode. Indium tin oxide (ITO) often is used for the anode.
FIG. 1 is an example cross-sectional view of an OLED. The glass substrate 102 supports a transparent anode layer 104. The hole transmitting layer 106 may be a carbon nanotube (CNT) based layer in some cases, provided that it is doped with the proper dopants. Conventional electron transporting and emitting and cathode layers 108 and 110 also may be provided.
When a voltage is applied to the electrodes, the charges start moving in the device under the influence of the electric field. Electrons leave the cathode, and holes move from the anode in opposite direction. The recombination of these charges leads to the creation of photons with frequencies given by the energy gap (E=hv) between the LUMO and HOMO levels of the emitting molecules, meaning that the electrical power applied to the electrodes is transformed into light. Different materials and/or dopants may be used to generate different colors, with the colors being combinable to achieve yet additional colors.
The technology has desirable attributes such as large viewing angle, fast response time, high contrast, and a Lambertian profile.
Although significant progress has been made on the electronic quality of the emissive and charge carrier layers, a significant portion of the light emitted is trapped by both the ITO coating on the glass and the underlying glass substrate, e.g., as wave-guiding modes promoted by interference effects. Because of this inefficiency, some of these devices are driven at higher current densities than what normally would be required. This unfortunately has a negative influence on their lifetimes. Even under these non-nominal driving conditions, the luminous efficiency of OLEDs can be significantly below that of fluorescent lamps.
It would be desirable to improve the light output of an OLED device, e.g., through a light out-coupling strategy. Doing so could improve the lifetime and/or overall luminous efficiency of the device. Several techniques have been proposed to improve the light efficiency, but these methods unfortunately do not meet the practical requirements of manufacturability.
As alluded to above, there have been several attempts to improve techniques for light extraction efficiency. For example, an attempt has been made to increase the extraction from the substrate into the air by way of adding micro-refractive or diffractive structures (e.g., arrays of micro-lenses or pyramids, scattering layers, etc.) to the substrate surface. Depending on the reflectance of the OLED stack, the extraction from the substrate into the air can be increased considerably, typically up to 30%. Unfortunately, however, these structures tend to be quite fragile.
Another attempt relates to monochromatic light emitting devices. In such devices, the angular distribution of the light, emitted into the substrate depends on the layer thicknesses of the OLED stack (e.g., by virtue of the micro-cavity effect). By proper design, the amount of light in the escape cone of the substrate can be increased and external efficiencies of up to 40% can be reached at the design wavelength.
Still another approach involves harnessing the “organic modes” that represent about 50% of the generated photons by the introduction of ordered or random scattering structures into the OLED stack. There is a drawback, however, in terms of a possible negative influence on the electrical performance, inasmuch as the anode would be rough, and localized current hot spots that are detrimental to device performance can develop.
A persistent challenge involves attempts at matching the refractive index of the glass substrate and the organic layers so that the organic modes are turned into substrate modes. The amount of light extracted into the substrate can indeed be increased by a factor 2-3, at least theoretically.
Provided that the OLED has a highly reflective cathode and is thick enough, 80% of the photons generated inside the OLED can be extracted into a high index substrate. However, the remaining issue is still then to out-couple this light into air without reverting back to one of the above-described strategies.
FIG. 2 shows different major light modes in connection with a schematic view of an OLED device. As can be seen, the major modes include a light in air mode (A), a light in glass mode (B), and a light trapped in the organic layers and/or the ITO. It will be appreciated that there may be more “B-modes” where the glass is thicker and/or more absorptive. It is noted that there also is another component related to Plasmon losses in the cathode, although this is not depicted in the FIG. 2 schematic view.
In view of the foregoing, it will be appreciated that there is a need in the art for techniques for improving the light emitting efficiencies of OLED devices.
One aspect of certain example embodiments relates to a light out-coupling layer stack (OCLS) on a substrate (e.g., on a glass substrate), with a view towards reducing wave-guiding modes.
Another aspect of certain example embodiments relates to scalable techniques for achieving higher luminous efficiency in OLEDs.
Certain example embodiments relate to a method of making an electronic device is provided. An optical out-coupling layer stack (OCLS) is disposed, directly or indirectly, on a substrate. A layer comprising a transparent conductive coating (TCC) (e.g., a transparent conductive oxide or TCO) is disposed, directly or indirectly, on the OCLS. One or more light emitting layers is/are disposed, directly or indirectly, on the layer comprising the TCC. A layer comprising conductive material is disposed, directly or indirectly, on the one or more light emitting layers. The OCLS comprises an isotropic layer matrix including an organo-metallic chelate hybrid material and a matrix core including dispersed scatterers. The OCLS has an index of refraction of at least about 1.8. The dispersed scatterers have a Mie-like scattering efficiency of greater than 1, leading to an index matching out-coupling efficiency for the OCLS of greater than 1.
Certain example embodiments relate to a method of making an organic light emitting diode (OLED) inclusive device is provided. A layer comprising a transparent conductive coating (TCC) is disposed, directly or indirectly, on a glass substrate. First and second organic layers are disposed, directly or indirectly, on the layer comprising the TCC. A layer comprising a conductive material is disposed, directly or indirectly, on the one or more light emitting layers, with the layer comprising conductive material being reflective. An optical out-coupling layer stack (OCLS) is disposed, directly or indirectly, on the substrate, with the OCLS including an isotropic layer matrix including an organo-metallic chelate hybrid matrix with scatterers dispersed therein. A relative refractive index m of the OCLS, a size r of the scatterers, and a concentration of the scatterers (1/s3) are selected so as to increase the total integrated light out-coupled from the device to a level where total out-coupling efficiency for the device is greater than it would, if no OCLS were provided.
Certain example embodiments relate to an organic light emitting diode (OLED) inclusive device is provided. A glass substrate is provided. A layer comprising a transparent conductive coating (TCC) is supported by the substrate. First and second organic layers are supported by the layer comprising the TCC. A reflective conductive layer is supported by the first and second organic layers. An out-coupling layer stack (OCLS) is interposed between the organic layers and a viewer of the device. The OCLS includes a hybrid organic-inorganic polymer matrix having scatterers dispersed throughout in a manner such that each scatterer is located in the far field of its nearest neighbor. The scatterers are sized, shaped, and positioned relative to one another so as to (a) have a high Zeta potential, and (h) promote Mie-like scattering of light passing through the OCLS.
These and other embodiments, features, aspect, and advantages may be combined in any suitable combination or sub-combination to produce yet further embodiments.